1. Field of the Invention
This invention relates generally to the field of building construction and in particular to structural framing elements for building construction.
2. Description of the Prior Art
Buildings are subjected to many forces. Among the most significant are gravity, wind, and seismic forces. Gravity is a vertically acting force, wind and seismic forces are primarily lateral (horizontal). Many buildings use shearwall diaphragms or panels to resist lateral loads. A shearwall panel is formed by the application of one or more types of sheathing such as, plywood, fiberboard, particleboard, and or drywall (gypsum board), to the inside or outside or both sides of a wall frame. The sheathing is fastened to the wall frame at many points creating a shearwall diaphragm or panel. Many suitable fasteners are available and nails are commonly used and will be referred to hereafter. The sheathed shearwall panel is used to conduct the lateral force acting on the frame of the building to the foundation.
Buildings require a strong base for support. Most buildings have a concrete base that is generally referred to as the foundation. A concrete pad whose top forms a continuous plane from edge to edge is called a slab. With a slab the concrete forms the floor of the building. The deepest concrete support that follows the perimeter of the building is called the footing. In a building without a concrete floor, the floor may be supported by short concrete walls called stem walls that are supported by the footing. Some grading considerations or design requirements necessitate a hybrid of a slab and a stem wall. This results in the use of short concrete walls extending from a few inches to a few feet above the level of the concrete floor. Foundation will be used hereafter in place of stem wall, footing, and slab.
The site where the building is to be erected is first graded (leveled). Wooden boards are nailed together to create a ‘form’ or mold for the foundation (slab, footing, stem wall). The forms mark the edges of the foundation. Next, wet concrete is poured into the form and the surface is smoothed and the concrete is allowed to harden. As the concrete hardens, bolts are partially imbedded in the top of the foundation with the threaded end of each bolt protruding out of the foundation. The bolts are embedded wherever a wall will contact the foundation/stem wall to provide a means of securing the wall to the foundation.
The frame of the walls is fabricated next. Each wall frame section is composed of several elements. In North America, the wall frames of most homes and small buildings use wood or metal elements having cross sectional dimensions of 2″×4″, 2″×6″, or 2″×8″. At the base of a wall frame is an element called a mudsill, and wood or metal stud elements are attached on top of, and perpendicular to, the mudsill. On top of the studs is a top plate that is secured to each stud. Holes are drilled through the mudsill for the foundation bolts to pass through the mudsill. After the wall frame elements are connected together, the wall frame is tilted to a vertical orientation. The wall frame is put in its finished location with the foundation bolts protruding through the holes drilled in the mudsill. Once adjacent wall frames are in place, they are secured together at the corners and an additional plate (top cap) may be added which overlaps the top plates of adjacent wall frames.
After the building frame is completed, the building is ready to be sheathed. Conventional building construction uses sheathing inside a building (drywall) which forms the wall surface which we all see, and sheathing on the roof which helps keep the building dry. Plywood or other sheathing is also applied to the outside and sometimes the inside walls of every building. The panel created by many fasteners attached through the plywood or drywall into the supporting wall studs, mud sill and top plates creates a sturdy vertical diaphragm known as a sheathed shearwall. Drywall or gypsum sheathing provides insulation and fire resistance and some structural stability. The structural contribution of a drywall panel is limited because of the relatively delicate composition of the drywall. Where higher lateral force resistance is required, builders and designers generally use plywood or particleboard or fiberboard or metal sheathing fastened to the wall frame. Plywood is the most common choice and will be discussed hereafter, but other suitable materials may be used. Plywood is available in 4′×8′ sheets that vary from ¼″ to over 1″ in thickness. Plywood is composed of many thin layers of wood glued together under pressure with the grain pattern of adjacent layers perpendicular to each other for strength.
Review of damage following the Northridge earthquake, revealed that many plywood sheathed shearwalls failed under the seismic forces. The nailing of the sheathing in the field during construction leads to many failures. Nails driven through the sheathing miss the frame member they were intended to penetrate creating ‘shiners’. Nail heads penetrate the skin of the sheathing during nailing which weakens the sheathing and allows the nails to be pulled through the sheathing under load conditions as well as inducing failures in the integrity of the sheathing. Shearwall fabrication requires regular nail spacing of 3″–12″ depending on the design requirements. Current field fabrication techniques are not sufficiently accurate to consistently meet the design specifications. Therefore every shearwall panel may be nailed differently and many may be installed with fewer nails than required to handle the required design load.
The rise in land prices has caused the building of more multiple floor dwellings to raise housing density. Multiple floors significantly increase lateral loads and thus increase the use of field fabricated sheathed shearwalls. In many multiple story buildings the entire outside of the building may be sheathed.
Consequently, many building departments may be limiting sheathed shearwalls to a maximum height/width ratio of 2:1. Where walls are typically eight feet high, the minimum shearwall width would be four feet. This restriction has implications throughout a building. At the front of a garage narrow shearwalls, two to three foot wide, are common. Narrow sheathed shearwalls are also common adjacent to window and door openings.
The interface between the shearwall and the foundation may also be area of weakness. The conventional practice of locating holdowns within the framework of a sheathed shearwall weakens the sheer wall and the frame-foundation interface. Bolts imbedded in the concrete of the foundation provide attachment points for the walls and shear panels. These bolts are intended to pass through the mudsill of the sheathed shearwall to prevent lateral movement between the sheathed shearwall and the foundation. The foundation bolts also transfer the lateral load from the top of the sheathed shearwall to the foundation. Quite often the bolts which are supposed to secure the walls and shear panels are placed several inches away from where they are required for optimum load transfer and ease of wall construction due to inaccurate measuring and carelessness during field installation of the bolts. The resulting misalignment forces some of the framing members to be trimmed to fit, or in some cases, the intended foundation bolt must be cut off and an epoxy bolt or a “red head” must be used. The resulting attachment of the wall to the foundation is a potential point of failure.
Another common fabrication error is oversize holes in the mudsill. The mudsill is the base member of a wall frame that is in direct contact with the foundation. Many different causes result in holes in the mud sill which don't line up with the bolts placed in the foundation or in the stem wall. This requires extra holes, or oversize or elongated holes be created in the mud sill which may weaken the frame-foundation interface.
The attachment hardware that may be used to connect a shearwall to the foundation may be another point of weakness. If a field-fabricated shearwall were ever built in exact compliance with the design, the attachment hardware would likely fail before the shearwall. In most cases the attachment hardware is fabricated by folding steel strips with a few tack welds. In practice the folds provide the necessary flex in the attachment hardware to induce failure. In other cases, the method of attaching the attachment hardware to the studs induces cracking of the studs.
What is needed is an alternative to field fabricated wooden shear walls that has the ductility and simplicity of wood construction combined with the higher load capacities and consistency of steel products.